Jack Russells are like a full sized dog crammed into a tiny package, they have no fear whatsoever :)
Absolutely. Most terriers are. I had a mixed terrier once run a full-blood rottweiler off our porch. Damned dog looked so confused, but he took off running with her on his heels, barking her head off.
The female Jack Russell I have now will wait until the Pittweiler is asleep and then jump up and mount his head and start humping his head like there’s no tomorrow.
Friends of ours have a JR/Chihuahua mix. It got the hyper-spaz of the JR, and all the afraid-of-the-doorbell-so-I-have-to-whiz-all-over from the Chihuahua. That's fun to clean up after.