I love playing banjo with good fiddle players, and to improve my banjo playing, I took up fiddle 7 years ago. It's given me a better way of listening to fiddle tunes, and definitely improved my banjo playing. Now, if I'm teaching banjo it's great to play the tune on the fiddle and have my student playing banjo with it. Cheers us both up, he's less concerned about playing exactly what I'm playing and more with making the tune sound good.
To learn fiddle (or anything else) it helps to play along with someone playing a different instrument.
Ray