Robert Mitchum's 4 syllables (2 + 2) hit the Hollywood sweet spot.
Robert's 2 beats vs. Mitchum's 2 create a natural cinematic snap.
Robert Mitchum's softer consonants cost it screen presence.
Robert Mitchum misses the alliteration hook that makes names stick.
Robert (6 letters) and Mitchum (7) are balanced in length — clean on any poster or marquee.
Robert Mitchum has mostly familiar phonemes with a few tricky patterns.
Robert Mitchum has decent vowel richness, but stronger A and O sounds would lift it.