All right, one of my very, very first AH answers was:
With trepidation, I offer you this sacrifice...
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Historical background: English alehouses, specifically, undergo a transition in the later 14th century. Brewing ale was a fairly standard in-home chore--ale was part of the daily diet. At the beginning of this time frame, ale houses were most often literally "houses that sold ale." A family with another baby on the way or compensating for the loss of a valuable livestock animal would brew extra ale one week, put up some kind of sign outside to notify fellow villagers or townspeople of ale for sale, and hope they reeled in customers looking for a drink in good company. (Medieval ale had a very short shelf life.)
Towards the end of the century, alehouses start to become more permanent establishments alongside their wine-, food- and lodging-providing counterparts. I'm not personally familiar with a decisive argument for the shift, although the drop in grain prices around that time would have made mass-brewing of ale significantly more economically feasible and potentially profitable.
In the earlier part of the century, ale was sold as often for consumption off premises as in the alehouse, so we can safely assume payment before drinking in those cases. Additionally, if you were simply popping into your neighbor's house for a drink and a chat (or a knock-down, drag-out barfight!, cf. Bennett, 20) at the end of the day, it would be pretty hard to get out of paying one way or another.
With more permanent establishments--and here I'm going to broaden the scope to taverns and inns as well--court records can be ambiguous as to the order of service and payment, although it seems like payment came last. Ruth Karras has a couple examples of a "tavern guest" meeting up with an accused prostitute and her pimp:
"The ostler conveyed her to the George into a chamber there and after came the priest...they went to supper all three and the priest paid for all." (*Common Women, 72)
Payment for lodging and food after the fact seems to be supported by literary sources--in the 12th century French Erec et Enide, Erec pays the duplicitous host on his way out the door; the conniving tavern host-priest (...always a priest) in the fabliau Le prestre et le chevalier lays out a lavish meal to the knight's delight--then proceeds to charge him for each item down to individual flecks of pepper.
Indeed, these literary sources paint a pretty grim picture of the public reputation of hosts-for-profit--and alehouse keepers are no exception. If anything, the situation is worse: they're a frequent character in artistic depictions of hell, and two of the seven personified Deadly Sins in the English poem Piers Plowman are associated with brewing ale.
Which is to say, take the source record on alehouses for what it is: people dealing with a vital, omnipresent, but morally ambiguous aspect of their world, with all the biases that entails.
That said, when ale-sellers show up in court records (at least, for reasons unrelated to licensing), it's generally because of customers' accusations that the sellers cheated them on measurement. This probably reflects a combination of alewives as an easy target with the alehouses' roots as a house: your "cup" or "glass" of ale was probably any old container the owner had on hand, so standard measurement was a judgment call. Still, though, the general paucity of reverse accusations--sellers suing customers for lack of payment--suggests that brewers were pretty good at preventing people from skipping out on their payment. (Also, if people skipped their tabs, presumably they wouldn't be suing over dishonest measurements.)
One of the most well-known satires of ale sellers, John Skelton's The Tunning of Elynour Rumming, portrays an alewife who squeezes every last bit of payment out of her market. She chases away people who can't pay
With a 'Hay, dogge, hay! Have these hogges away!'
With 'Fo, ther a stench--Gather up, thou wenche
Take up dyrt and all
And bere out of the hall
collects payment in anything and everything that people can offer (even encouraging customers to go into debt):
Anone cometh another, as drye as the other,
And wyth her doth brynge mele, salte, or other thynge,
Her harvest gyrdle, her weddynge rynge
To pay for her scot as cometh to her lot
and, um, enriches her ale with a secret ingredient:
And somtyme she blennes the donge of her hennes
And the ale together; and sayeth, Gossyp, come hyther,
This ale shal be thycker and flowre the more quicker.
Obviously, or at least one hopes, that is satire, and your good neighbor wouldn't sell you chickenshit in exchange for your wedding ring. Although Elynour mostly comes out on top in the poem (which is very English--in contemporary German literature, transgressive women never win), the poem suggests that the people she's kicking out for lack of payment have already consumed at least some of her ale:
With, Get me a staffe--the swyne eate my draffe
Stryke the hogges with a clubbe,
They haue dronke vp my swyllynge tubbe
Again--it's satire. But if we take the usual way of interpreting satire as rooted in reality so as to seem realistic though exaggerated, at least some of the time, alehouse guests by the end of the Middle Ages were drinking up before paying up. This would reflect the norms already established by taverns and inns, back in the days when alehouses were transient neighborhood affairs.
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Sources: Judith Bennett, Ale, Beer and Brewsters in England, 1300-1600; Ruth Mazo Karras, Common Women: Prostitution and Sexuality in Medieval England; Andrew Cowell, At Play in the Tavern: Signs, Coins and Bodies in the Middle Ages; go read Elynour Rumming, it's fantastic
ETA: to credit Skelton
This one is a little more to the present question, but lacks the most excellent poem:
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This question is definitely more situation-dependent than the fantasy model of "Mos Eisley cantina^1 with elves" would suggest. That means time and place within the Middle Ages, of course, but also individual case.
...Naturally, I am going to focus on probably the closest equivalent to said cantina, which is taverns (a.k.a. public houses), and mostly in the Holy Roman Empire.
For taverns, we're talking about a permanent establishment dedicated to selling alcohol and probably some form of food. These are places where, if the owner/operator was male, said owner/operator would join the publicans' guild. In general, they did not have the upstairs room for the Nazgul to not kill the sleeping hobbits. In occasional cases? Ehhhh well.
The most important obligation of taverns was, of course, to get their patrons sloppy drunk supply alcohol. In the late Middle Ages, distilled liquor was the province of apothecaries and alchemists; German public houses sold beer and wine.
In the early modern era, some towns mandated that tavernkeepers pick one or the other. But then we have cases like Oberwainbach in 1521 (STILL THE MIDDLE AGES), which attempted to require taverns to sell multiple kinds of (good) wine as well as good beer.
To get the alcohol they sold, beer-providers could brew their own or purchase it. Wine-providers would have to buy it. But we're not talking about the kind of class or classiness divide you might envision today. The wine-growing region of Germany was MUCH bigger in the late Middle Ages than it is today. And, well, those extended cultivation areas produced lousy wine.
There absolutely were class divisions among tavern patrons. And not just how we'd think of patrician/bourgeois/working class/scraping-by-or-not today. A tavern's market could be as specific as members of a single craft guild.
Publicans (women and men) also had quality requirements. Or at least, quality:price requirements, and at least in theory. From the 13th century, for example, Burgdorf tavern-keepers were required to provide samples of their product to official city tasters, who would tell them what price they could charge for it.
The job of city taster rotated annually. I would have liked to have been there for those city council meetings.
Public houses did also provide food. In the sixteenth and seventeenth centuries, which in general placed more emphasis on social regulation and order through law, cities passed laws limiting the types of food that could be sold outside of main meal hours. That was obviously an attempt to get people to go home earlier, ergo drink less and not get disorderly drunk. The need to pass this law suggests that tavern-keepers were rather fond of keeping patrons around longer.
Oh, and on that pesky subject of keeping order.
We have plenty of surviving examples of barfights from the late Middle Ages, which we mostly know about when they ended in murder. Said murders frequently took place in the streets outside, which suggests that in many cases, publicans threw some of those drunk and disorderly patrons out of their taverns.
Unfortunately for present purposes, everyone's favorite example involves (at least in testimony) no action by tavern owner Alice de Wautham. Fortunately, I'm going to tell it anyway. It was 1301, on the verge of spring. Thomas de Bristoll and Joice de Cornwall (both English; both men) were playing checkers when a group of men decided the checkerboard was the perfect place to have sex with the prostitute they had hired.
Thomas got understandably mad; Robert de Exeter, one of the interrupters, somewhat less understandably stripped Thomas down to his underwear.
This continues with Thomas escaping upstairs, Robert assaulting Joice instead, Joice escaping outside...and Robert brutally stabbing him to death.
(I warned you these stories don't have happy endings.)
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^1 Strictly speaking: pop culture stereotype of US "Wild West" saloon, but humor me, Rise of Skywalker opens in ONE MONTH.
In medieval Ireland, this role was filled by the briugu, a wealthy man who had chosen to raise his status in society by providing food, drink, and a place to stay to any who entered his doors. He was supposed to already have at least a hundred people working for him, and a hundred of every kind of livestock. He did not take payment - the ability to offer hospitality was important. Many Medieval Irish people would not have travelled far from home, so the briugu would probably mostly host warriors, poets, judges, clergy or kings (of which Ireland had about 150) and their retainers.
In the Ulster Cycle tale “The Destruction of Dá Derga’s Hostel” the briugu Dá Derga’s home has 7 doors which are never locked, he hasn’t taken the cauldron of boiling food off the fire since he began, and he is fostering two sons of the king of Leinster. He has many rooms full of peculiar guests and they get into a battle and, well, as the title suggests, things don’t go well. This portrayal is of course fictional but it speaks to the social importance of the briugu.