I'm considering a scene that has a couple of people having to lay in wait for a bad guy.
I'd like to understand how the practicalities would be addressed.
I'm assuming things like clothing and equipment and the site would be vegetation/underbrush.
How are bodily functions coped with? With only two people, what would be the best overwatch strategy? What would be the longest duration for the exercise?
You're looking for a military sniper most likely. I would recommend the book "Carlos Hathcock, White Feather, USMC sniper" for research purposes.
Well just to put my own two cents in and since you have me wondering if you're a long lost relation and since i've already tried messaging you about this, a little background on what precedes this ambush scene would help a little.
I mean knowing how the characters got to needing to ambush their enemies would help to know how to comment on it.
Everything depends on who you are and who you're hunting.
Is this an urban environment? Suburban? Rural? Estimated range to target? What are the chances of a retaliatory strike? Are there police able to respond to a call about someone shooting? Are the snipers professionally trained in any capacity?
As for bodily functions, that depends on the threat level at the sniper's nest. If you are danger-close to the target, you can't move. If there are dogs that might smell you or see you, you can't move. If you have to wait for days before the target shows up, you can establish a hide well back from shooting position and maybe have more freedom of movement, but you still have to allow for staying concealed.
The professionals have chemical powders they spread around their hides to mask human scent and dissuade dogs from wanting to investigate. And, yes, some times you have to kill a person or dog that gets too close, putting them in a bodybag and covering them with the powders that mask scents.
Keeping watch is usually done in three or four hour shifts so whoever's on duty doesn't get too tired and inattentive.
Hathcock's story is chock-full of good tips and tactics, well worth having in your personal library!
With today's weaponry and other tech, you could "easily" stand off a mile and still have a very good chance of making the hit. Even in an urban environment, getting very very long shots is possible if you have elevation you can take advantage of.
One of the big problems with urban hunting is getting access to a place where you can set up. I know all the movies show the sniper on a rooftop, but that's hardly realistic since the roofs are usually very secure parts of the building just so the average idiot doesn't go up there and cause problems. There's also a very good chance of being spotted by people in other buildings unless you're the highest... and that's not always possible.
Similarly, cutting a hole in a window sounds nice, but it's not very realistic. The winds at that elevation are a nightmare to deal with, especially when the bullet gets that first taste as it exits the building and gets hit by crazy crosswinds coming at it from all angles.
I always get a laugh when I see a scene where the sniper looks at some moving flag to judge the wind where the target is standing. While that's nice, it doesn't mean much in an urban setting because there's a thousand changes in the winds before the bullet gets to the target! Every skyscraper along the trajectory will cause turbulence of one kind or another.
Pro's do make shots like that all the time, but it's the mark of a pro that it can be done reliably. If your characters are pros, no problem.
Any sniper scenes in movies are just a bare taste of the real world, obviously, but the part that always interests me is the before and after stuff. Setting up and then exfiltrating the area, without being seen, that's the real test of skills. I get a serious kick out of seeing what people think up.
As for how long you wait on target... that depends entirely on the story. Waiting three or four days isn't at all unusual. Your intel might say that Bob's going to be there on Tuesday, but you can never tell for certain and things happen to change a bad guy's schedule just like anyone else's.
You might have to get on site and dug in a week before he's scheduled to show up just so the vegetation can recover and rain can help mask your scent. If it's a high-value target, security is only going to get tighter and tighter as his arrival time approaches, so getting in early is usually the only option.
Waiting for three days in a torrential downpour isn't the best of times, but it's great for covering your tracks.