I’m guessing Christopher Meloni or William Petersen. Am I in the ballpark?
I pretty much did. So I guess you have a point. I guess the delusion of some online anonymity.
Oops. I guess I guessed wrong
I was once an extra in a Hollywood movie that was shooting in my town, and I vaguely remember that they were also looking for people among the pool of extras to serve as stand-ins.
I don’t actually remember seeing any stand-ins during shooting; I was sitting very close to the lead actors during one of the scenes I was in and it’s my recollection that they were present for most if not all of the shoot. This was an outdoor scene using natural lighting so that may have simplified things, and it may also be that I’ve either forgotten stand-ins being used when things were first getting set up or that I hadn’t been put into position yet when that was happening.
I was right!!
Yeah, I didn’t want to give anyone the impression it was a great movie.
Yeah- way off base.
Yes, although it’s SAG-AFTRA now. An actor can also earn their SAG-AFTRA card by being hired for three days of union background work. However, not every extra on a production is union, there are minimum numbers in the contracts (something like 45 for movies and 20 for tv shows), and usually already-union actors are hired for those slots.
We also get paid for showing up for costume fittings, and get paid more for having your car used in a scene. Many times I was booked specifically because they wanted my boring blue econobox in a scene - sometimes I wasn’t even used, just my car! Ah, good times. 
So did I. I got Christopher Meloni, but picked Twelve Monkeys. Even though his part is small, I figured he may have required a stand-in at least occasionally.
This refers to another frequent role for stand-ins: they often do nude scenes for an actor. A general rule of thumb is if you’re watching a nude scene and you never see the actor’s face and nude body in the same shot, they probably used a stand-in for the body shots.
Another job for stand-ins is when an actor doesn’t have some skill that’s needed for a scene. It’s not stunt work but maybe you have an actor who can’t dance or pitch a baseball or ride a horse. In some cases, they’ll just use a stand-in for the scene and edit together footage of the actor and the stand-in.
None of those folks are called stand-ins though. They are called body doubles or stunt doubles or special skill doubles.
There are also hand doubles, foot doubles, etc. I was once interviewed (really just a look-see but they call it an interview) to be a hand double for Sissy Spacek. There were about four of us, they looked at our hands and chose one of us (not me) to stay and shoot the scene. I got paid for a quarter of a day just for showing up.
Timely thing: Somebody binge watching Friends noticed two scenes where stand-ins were visible. One with Aniston’s, the other with Cox’s.
They are also used during filming but not intended to be visible. So that the other actors have someone to look/speak to while the original actors are resting in their trailers.