Pass Blocking

Is it possible to explain to those of us who don't know what it takes to make this game work,

what determines whether a TE, FB or RB runs a route or remains as a pass blocker from play to play?

Can we expect one or the other when building our lineup?

Basically, the more players that the defense rushes, the more players that stay in to block. WR always run routes. FB is more likely to stay in to block than HB, and HB is more likely to stay in to block than TE.