The child was required to be seated on Sizemore's lap during the photo session. This is when he allegedly indecently assaulted her.
"The girl was so young it was unclear to her and [later] her parents what had actually taken place and if it was intentional or not," production manager Cassidy Lunnen told THR.
Another actor, Robyn Adamson, was present for the shoot but not in that shot, and watched the young actor as the alleged incident took place.
"At one point her eyes got just huge, like she could've vomited ... Later, when I was told about what happened, I knew exactly what it was," she said.
By the next day, the girl had told her mother what had allegedly happened - and word soon spread throughout the crew.
Production assistant Roi Maufas said that Sizemore "was this guy who was already known for making inappropriate comments, being drunk, being high. We're talking about consistent behaviour, just being 'Tom Sizemore' on set every day. Then this happens. Guys reached for hammers."
Sizemore vehemently denied the allegations when confronted with them on set, and the girl's parents did not press charges. Now, in the wake of the tide of sexual assault allegations going public throughout Hollywood, she is reportedly pursuing her legal options against both Sizemore and her parents.
Sizemore, 55, is best known for supporting roles in films including Saving Private Ryan, Pearl Harbour and Born on the Fourth of July.
He's also made headlines for his frequent personal and legal problems and has previously been charged with domestic abuse and drug use.