We call them Spotters in our garrison. The roles differ depending on the event, but you can be expected to help assist troopers getting their armor on and off (limited mobility of the costumes means that they can be difficult to put on unassisted).
If it's a meet and greet where the characters are walking around then you'll want to stay with them and help point out to them when they need to walk up and down curbs, or point out electrical cords on the ground so that they don't trip. Sometimes little kids will be trying to talk to a trooper and the trooper is unaware because the kid is standing right in front of them where their helmet blocks their view, plus they may not be able to hear the kid. The handler can get the attention of the trooper and point down and tell them that someone is trying to talk to them.
At hot events, you'll need to regularly ask each trooper how they are doing and if they need any water or need to be escorted back to a changing area to take a break.
Sometimes large groups of people will gather and want to take their photo with the troopers. A handler keeps an eye on who has been waiting and tries to direct traffic and make sure their isn't chaos but some order to make sure everyone gets a turn.
If their are kids who are pestering and/or poking the troopers you can shoo them away or ask them to stop, give them the evil eye etc.
Sometimes the public will have questions about the garrison/legion/troopers and the handlers can answer these questions as it is much easier for them to carry on a conversation since they aren't wearing a helmet and can make eye contact and can hear clearly.
You can also bring your camera and take candid shots of the event to document it, and then share the pics with the garrison.