Started playing guitar at 11 years old (33 now). Several years later, was in a band with some high school buddies and we had three guitarists but no bass player.
Being a McCartney fan (and aware/appreciative of how the bass can propel and add to a song), I "volunteered" to be the bass guy, and immediately rented a cheap Precision Bass knock-off from Hondo (in canary yellow, no less) and proceeded to learn to play bass (1s and 3s, with fingers, etc.).
Cool thing is, they both helped the other: learning bass made me a better guitar player (timing, rhythm, etc.). Being a guitar player made me a better bass player (I know the neck well enough that I could boogie around with McCartney/Entwistle ease, and wasn't afraid to play two or more notes together sometimes).
I know just enough music theory to be dangerous (how chords are constructed, what makes chords major, minor, sevenths, diminished, major sevenths, etc.), and also know the notes on the frets, so that helps too.
Can tinkle around on a piano okay. Know how to chord and play basic, clean progressions and can do a pretty decent (although slighty half-ass) Jerry Lee Lewis things (walking boogie left hand and hammering, rock 'n' roll/"Chuck Berry on piano" licks with the right hand.
That's it: guitar, bass and SOME keyboard.
Oh, and mandolin.