A senate meeting required a quorum of members, and very often this was not possible. In other words: almost never was the full contingent of senators present in the city or willing to attend a meeting, and never was the number anything like 900. More like 300, depending on the time period and the faction currently in power. Technically you could be punished for not attending a meeting, but this was rarely if ever enacted.

There was strict procedure during the meeting. Senators spoke in order of their "rank," which is very difficult to explain here. The consuls and consuls-elect usually were on top, then the princeps senatus, then any and all consulares, then propraetors and current praetors, and so forth, all the way down to the lowest of the ranks. If the topic was very important, every senator present would be compelled to speak. The meeting had to conclude by nightfall for religious as well as practical reasons, and yes, filibusters were possible (like Publius Clodius against Cicero's efforts, e.g.). This was called diem consumere, to "eat up the day." Probably most of the time, lower-ranked senators did not launch into long diatribes when asked to speak. The senators, like all the rest of Roman society, were divided into complicated hierarchies, with subtle and not-so-subtle social ties to one another, to factions, to causes, etc. Many probably said simply "I agree with X," as Cicero hopes to do when asked about Caesar's impending crossing of the Rubicon: adsentior Gnaeo Pompeio "I agree with Gnaeus Pompeius." Virtually like saying "aye" or "nay" in modern parliament.

Having said that, any senator had the right to "wander" from the topic at hand, if he wished. It would be very unusual for a lower-ranked member to do so, but the powerful men could and sometimes did, especially those full of hot air (like Cicero). I get the impression that often senators were lured to meetings on the pretext of some minor issue (deciding to fix a road, or something) and then ambushed by a much larger issue.

The senate of the Empire was eventually little more than a collection of yes-men for the Emperor, but not so during the Republic. The senate could and often did act contrary to powerful Roman statesmen like Marius, or Pompey, or Caesar. Part of the system of checks and balances in the Roman government stipulated that the senate not be compelled to act as this or that powerful man wished. When it did happen (for instance when Sulla marched on Rome and held a senate meeting "by the sword"), the decisions were usually quickly overturned afterwards. From Cicero we get an impression of the senate, roughly 60 to 44 BCE, which was rather cantankerous, recalcitrant, and downright stubborn, even as the Roman world descended into armed contests between powerful principes.

Membership in the senate was not technically for life. Senators were expected to maintain a certain level of good character (whatever that means), and, at least after Augustus, a certain amount of assets. The property qualification might have existed prior to Augustus, at least in spirit, but the evidence isn't good to prove or disprove that idea. It was the magistrate(s) called the censor who was and wasn't enrolled in the senate (the patres conscripti), and that magistrate had the power to remove those who were not living up to their duties.

Edit: as always, for more on the Senate (and other Roman governmental things), see Lintott, The Constitution of the Roman Republic (Oxford 1999/2002).