Shelley Duvall Took One Final Gig to Lift Her Spirits as Health Declined

Shelley Duvall returned to acting for the last time after two decades away as her health rapidly declined ... 'cause she knew it would offer her unique solace, TMZ has learned.

As we reported, the trailer for the gory indie flick "The Forest Hills" dropped on Friday ... and fans were both psyched and bummed -- seeing the iconic actress in one last flick after her July death.

Her longtime partner Dan Gilroy tells TMZ ... she stepped in front of the camera again because she wanted to feel like an actress a final time -- knowing it would help lift her spirits amid her health battle.

Shelley -- who died at the age of 74 a few months ago -- had serious reservations about doing another horror film ... after all, she starred in the horror classic "The Shining," discussing the difficult filming process over the years.

Ultimately, she decided to take the chance -- and as Dan tells us, "She really rose to the occasion."

Shelley had limited mobility during filming and used a wheelchair ... but, the crew wanted to work with her so desperately, they brought the set to her Texas home and worked her lack of mobility into the character.

Dan tells us he is super grateful she had the experience ... and he found it slightly humorous that her final film was horror, when she most loved making family-friendly projects such as "Popeye" and "Faerie Tale Theatre."