Val Kilmer had been dogged by rumors that he was a Hollywood diva and a pain to work with ... but one costar and friend tells TMZ that's a total crock.

Danny Trejo, who was among the star-studded cast of "Heat" with Val, tells TMZ ... Val got a bad rap for being difficult to work with -- but the icon wasn't high-maintenance -- he was a "perfectionist." There's a big difference, DT makes clear.

As you sadly know ... the legendary actor died Tuesday at the age of 65 -- his death sending shockwaves through Hollywood as most didn't realize his health had deteriorated so severely.

Danny wanted to set the record straight, that Val was cool to everyone on a film set -- he simply believed if something could be done better, he wanted it done that way. And that ain't the same as a star being a blowhard for ego's sake.

Highlighting what a thoughtful dude Val actually was, Danny shared one of his favorite memories from their time on "Heat" -- and it's kinda a tear-jerker now.

One day, Val showed up to set wearing a fancy and obviously very expensive leather jacket -- and Danny commented on how much he liked it. The next day, there was one in his trailer ... a gift from Val.

Danny says the only issue he ever had with Val ... his pal didn't get nominated for an Oscar as Doc Holliday in "Tombstone."

RIP Val ... We'll be your Huckleberry.