LeBron James -- FIRST DAY ON MOVIE SET ... In New Amy Schumer Flick

LeBron James' movie career is officially underway ... with the NBA star roaming the NYC set of the new Judd Apatow flick he's filming with Amy Schumer ... and TMZ Sports has the footage.

Back in May, Bron secured a cameo in the movie "Trainwreck" -- a movie that's been getting a lot of buzz ... in part due to the fact producers are desperately trying to keep the plot under wraps.

Unclear what LeBron's role will be in the flick -- but he's not the only jock expected to make a cameo ... earlier this month, WWE superstar John Cena was spotted on the set.

Tilda Swinton and Daniel Radcliffe are also slated to appear in the movie -- we're guessing they'll have bigger roles ... since, well, they're professional actors.

LeBron's pregnant wife also made an appearance on the set. They haven't said when the baby is due ... but we're guessing some time in the next 9 months. Congrats!