In all the movies and stories there are barbed wire fences and barricades set up in between trenches. Are these accurate depictions of battle scenes during WWI? I cant imagine soliders would run out in the middle of combat to construct a barbed wire fence. Were they set up prior to digging the trench?
Yes, it is an accurate depiction...ish. Because when you watch depictions of WWI it's generally not the rote day to day lives; it is especially not some random battalion far away from the major offensive of that year. It's usually the men in the dead center of the action and thus with the most robust defenses and it creates this illusion that it was like that everywhere, which is frankly not true. That's not to say it wasn't a dangerous job but it should be taken with a grain of salt.
With that said however, regardless if you were in a 'hot' or 'cold' area, most time was devoted to maintaining and improving defenses. The major component of this was barbed wire entanglements. I think you're being a bit dismissive of the prospect of sending men out to maintain barbed wire -- it was the most invaluable tool in the immediate defense of a trench. It was near invulnerable to artillery and bomb (ie: grenade) fire and could utterly immobilize a man simply by snagging onto his pants or god forbid his flesh. It also touches on what i spoke of previously: the war was not some 24/7 all out offensive in every corner of the front. In fact in most of the front, especially during 1915-1917, if you were not in the immediate area of your nations offensive or where your enemy was hitting in that isolated region of fighting you were generally spared of intense fighting. To be frank, the average soldiers experience was 1-3 days of actual engagement out of the year and the rest either being shifted around the front or on rest and, if on the front, taking sporadic shell fire and maintaining trenches and scouting with small scale raiding.
Regardless onto actual ways they built up the barbed wire. Well it's a big heaping pile of 'it depends' but we got some good accounts of how it was done. Here's an account of one Lt. Barnett who actually did his duty under German harassment:
Last night we had to do wire again. There was a brilliant moon, and they put the machine gun on us! I had a stream handy and fell in gracefully. I sat in it for some time and improved the shining hour by winding barbed wire all in and about it. Then I sent the section [15 men] back and carried on with two sergeants, but they all turned out and fired volleys at us, so we solemnly crawled home on all fours. Later it got a bit cloudy and I went out with the captain, and we worked till 5 in the ironing and got lots done. They did not snipe at us very much. We had a little panic about a German patrol, but it soon blew over. Slept late this morning. Shelling and sniping pretty heavy to-day, but former not on this trench. I'm very well and happy.
. . .
Last night we had to work at the wire again, but it was not a bit exciting, as the moon was clouded over. We could hear Johnnie German doing the same 350 yards off, knocking in posts and fixing up (as we an see this morning) a most awful fence.
So basically as you would imagine. They would crawl out at night and maintain the barbed wire after the days damage from enemy artillery and maybe even raiding. Even moreso during major offensives. This would be done under sporadic enemy fire and while dodging enemy patrols at time. In more major areas this would be performed under the cover of the rest of the platoon to counteract enemy harassment via volleys. This being necessary because for the time the only way to put the stakes in for the wire was hammering it -- this was loud. At times men would, in their fear and trying to be quiet, not secure the stakes sufficiently:
The Company Commander who received Clarke and myself told us that he had done a great deal to strengthen the wire, and invited us to look through the periscope, saying that we should easily distinguish the new wire from the old. So we did, and saw the new wire clearly, but in front of the German trench! Our friends who had been holding the line had merely produced a number of ready made pieces of concertina wire and rolled them over the top, without troubling to fasten them down; and the wily foe, realizing that he had green horns to deal with, had come over during the night, abstracted the concertina wire, and fastened it firmly in front of his own trench.
In quieter sectors though it would get far more relaxed. In October 1917, in the then peaceful-ish Havrincourt sector, near Cambrai, there was an unofficial live and let live policy w.r.t. work in No-Man's-Land. This is accounted by Capt. Dugdale:
Time passed very peacefully as the Germans were very quiet. My battalion snipers had the time of their lives; never before had they been given such targets. We literally kept a game book of hits for the first three days; after that the Germans did not show themselves so much; also they started to retaliate.
Wiring was carried out nearly every night, but not in the style we were accustomed to in the days of the Somme. Our men did not creep through the wire carrying coils of wire, stakes, etc.; instead, a general service wagon was driven into No Man's Land with materials on board, which were dumped out when required. At first we expected bursts of machine gun fire every minute, but nothing happened. It must have become a well-established custom, as the enemy did the same thing themselves; we did not interfere.
Later in the war, the stakes would be replaced by metal corkscrews which could be silently screwed into the ground. However that did not stop men from being unnecessarily rowdy, usually via shouting or general clumsiness. Often a loud working party would meet their end when the Germans set up their men on the parapet and opened fire all at once, machine guns and artillery and all. However it would be an incredibly risky job throughout the war because of its reliance of going into no mans land. However the men were not unaccustomed to it as many jobs, including manning listening posts, offensive patrols, scouting parties, and raids were regularly performed by a platoon and all required silently maneuvering through the night.
^ these answers I've given previously should help.
I cant imagine soldiers would run out in the middle of combat to construct a barbed wire fence. Were they set up prior to digging the trench?
The trench systems grew gradually from late August onwards on the Western Front, setting down and extending as the fighting shifted further north from the Frontiers in the south, ultimately being largely complete by December 1914, though some of these positions were not always trenches as popular memory/imagination understands them. Barbed wire was laid when positions were step up, comprising often very deep fields of high, thick wire. Wire-laying and repair parties would be sent out to repair, fix, extend, etc these fields, often at night.
It must be noted though, that the image of constant fighting along the entire front is deceptive. Outside of major offensives, of which there were many in 1915, and two major ones in 1916 and 1917 each that tied up large numbers of men and guns, while there might be trench raids and spoiling attacks here and there, these often involved small units, and still took time to plan. By and large, life on the Western Front could be described as 'monotony punctuated by sudden violence'. There would be plenty of opportunities in quiet sectors or in lulls in the fighting to repair wire and carry out other various 'odd jobs' in the trenches. In the case of all armies, rotation systems moved units around, in the British case very frequently, so that your average Tommy might spend 40% of his time in combat of some kind or on the firing line at least, and the rest of their time in the reserve trenches, in billets behind the line or in quarters, in reserves, training, or transit to another sector of the front.