According to The Direct, James Gunn discussed the initial idea of having a Stan Lee cameo in Guardians of the Galaxy Vol. 3 before his passing. Gunn disclosed that his character Lambshank, a disturbing creature from the High Evolutionary's collection, was supposed to be portrayed by Lee. Due to Lee's advanced age, the role was planned to be a voice-only part as it would have been challenging for him to travel to Atlanta for filming.
I initially wrote the character Lambshank for Stan Lee, as I had written the movie before his passing. Knowing that Stan was getting older and it would be challenging to bring him to Atlanta for filming, I created a character that could be animated and have Stan provide the voice, with a resemblance to his iconic appearance. However, unfortunately, Stan passed away, and I deeply cherished the experiences of collaborating and directing him in numerous films.
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Source: The Direct