Its a fairly common trope in the movies, waiting soldiers being briefed about a dangerous mission with high casualities expected, then told that if they don’t want to volunteer for it they can leave the room and with no repercussions on their career. Of course no-one actually leaves and the super-secret briefing continues.
Does this actually happen in real life or is it just a movie thing? If it is real and someone got up and left would it really be overlooked or not?
Do not need answer fast.
It’s a movie thing - a shorthand trope designed to inform the viewer that this is Serious Business and build drama.
Well it’s sort of similar to SEAL training but not for a specific mission. A bell is present at all training. If at any time the individual feels the can’t or don’t want to continue they can go ring the bell and leave. There are no bad career repercussions and something like 85% wash out.
If you were dealing with professional soldiers it wouldn’t happen. They’re assumed to be willing to follow orders and do their best in dangerous situations. If someone wanted to opt out, he’d need to have a pretty good reason, or it *would *impact his career.
If it was a group of conscripts with low morale, who might not buy into the conflict in the first place (e.g. US in Vietnam), then you might want to get rid of anyone who was less than enthusiastic and might hang back and be unhelpful. But then of course there would be no career implications.
Being asked if they want to volunteer for a deployment is a real thing I saw many times - Guard, Reserve, and in my mixed Reserve/Active unit even among Active Duty troops. Some of those were in jobs that were above average risk but the expectation was still that most of them would come home. There wasn’t secret squirrel “we can’t tell you” either although sometimes we were missing a lot of information at the point we had to ask. There almost always were volunteers even when it was just put out to the group as a whole in order to alleviate the “I have to or my boss will think badly of me.” When I asked specific people I made sure to specify that I was asking with no pressure, strictly because I thought it was a good fit for their career and personal situations. I got more no answers than yes so a lot of those individually asked seemed to trust me when I said it.
I was in Germany during Desert Shield. My unit was not slated to go. The battalion commander had a formation and asked for volunteers to go in case there was a request for volunteers came down. I volunteered as did quite a few others. When the time came that slots for deployment came down that list was ignored. They cared about MOS and not who volunteered.
Thank you for the answers everyone 
It also happens in the military in real life. An example would be Marvin Shields, a Seabee deployed to Saigon during the war. He was assigned with a team to a Special Forces camp at Dong Xoai to construct a camp. When they came under attack, Shields was wounded by shrapnel. He was wounded twice more while trying to move other wounded men, and then:
He is the only Navy Seabee to ever have been awarded the Medal of Honor.
“Anyone who doesn’t want to, can leave” it has been a trope in literary or stage presentations of battle long before the movies:
Rather proclaim it, Westmoreland, through my host,
That he which hath no stomach to this fight,
Let him depart; his passport shall be made
And crowns for convoy put into his purse
(Henry V, Act IV Scene 3)
In one of Heinlein’s books, an inexperienced lieutenant on the front lines asks for and gets volunteers for a risky foray into no-man’s-land, and is berated for it by his extremely experienced sergeant. You never ask for volunteers; you pick the men who have the best chance of pulling off the mission, and then you tell them that they’re volunteering. Asking for volunteers just gets you heroes, and heroes are the last thing you want in an actual war.
The technical military term is “voluntold.”
“Volunteer” in the heat of desperate battle and “volunteer” for some prospective mission in the calm of a rear area are rather different things.
Which takes nothing away from Shields’ towering bravery that day. But does speak to the aptness of this example for the OP’s purposes.
Well, it’s a Bible thing. In one of my favorite Bible stories (Judges 7), Gideon has 33,000 men with him, and is preparing to attack the Midianites. He tells his army that anyone who is afraid can leave.
22,000 of them leave.