The superlative of believable is "most believable."
more believable, most believable
more most
I think the story about his parents is believable.
Drum-Believable was created in 2005.
I find it believable that Mark could have written that anonymous letter.
You have to have believable, logical characters who would have believable dynamics first. Then, you just put them together and write out what they'd do in that situation. If your characters are "real people" with backstory and emotion and personality, your dynamics will be believable.
Sleep is a noun and does not have a superlative. Sleepy is an adjective and the superlative is "sleepiest."
The prefix for "believable" is "un-", making the word "unbelievable."
The superlative for willing would be "most willing." There is no one-word superlative.
"Studying" is not a superlative, as it is not an adjective. "Most studious" is a superlative of "studious."
"Bib" is a noun and, as such, does not have a superlative form.
most superlative is the superlative form of superlative