Quick rules question - can you use skills that allow for rerolls on certain types of checks on other Investigators? So if someone else fails a Combat check and you have Marksmanship, can you allow them to reroll that check? If so, does that extend to the skills that give you an extra success in a particular type of check? What about Credit Rating - can you use that when someone else is purchasing something? And can you use the Bull Whip to reroll a die for someone else in Combat?

Short answer: no to all.

Slightly longer answer: Only abilities that specifically allow you to use them on other investigators (see Mandy) can be used on others.