For mandolin players, one of them is pick direction. Go down on the downbeat, up off the beat.
When you're strumming, this is natural. When you're playing a melody, lots of folks start out alternating pick directions each note, instead of each beat, and have to break a habit. To make this clearer, at the start of Happy Birthday, the right way is this
Hap py birth day to youbut they're doing this
Down up down down down down
Hap py birth day to youIf I see a mandolin player's right hand going every which direction, my just suggesting that he make his right hand a metronome can move his playing forward a long way.
Down up down up down up
If, however, you're already doing this because it comes naturally, and you can't imagine why anyone wouldn't, watch fiddlers bow. They change bow directions for completely different reasons, on purpose, and no two fiddlers bow the same tune the same way. It's perfectly normal for a fiddler to play several notes on a single bow-stroke.
Despite the difference, fiddle bowing is particular, not random. Most American fiddlers are down-bowers, who'll kick off a part by pulling their bows, and use a bow pull to emphasize strong beats. I've seen lots of Cajun fiddlers up-bow, pushing their bows for their strong beats. I've never seen the mandolin analogue, an "up-picker," which makes me inclined to try just to see how it changes the sound.
Because mandolins are tuned like a fiddle in standard tuning, many fiddlers cross-dress as mandolin players. They already know what to do with their left hands; it's their right hands that come them a cropper until they learn the trick of letting their right hands just mark the beat.