I think,
a) a great guitar player,
b) if there's some difference (originality) in his style, and
c) if there's musical and
d) personal chemistry between them,
e) if they do some decent rehersals,
f) add some degree of friendly, respectful competitiveness
can do wonders with Mark.
g) the guy should challenge him a little bit, force him to respond, take him A LITTLE away from his comfort zone
Sonny Landreth for instance.
Eric Clapton in Knebworth, where I think he "won" on away goal rule