You are a saboteur
Try to be helpfull in the beginning to avoid being a suspect.
When you have a helpfull path card, try to discard it, but don't do this too much. Else you will look like a suspect.
Try to create some chaos while not revealing yourself.
Lie about your goal card if you get caught by another player and accuse this player for being a saboteur. Now block him or her.
Near the end of the game, you must sabotage. Using a rock fall card on an important tunnel is a good plan. You can also block all other players (except suspect-saboteurs).
Three or four tiles from the end, you should act aggressively.
The last few connectors are where you can really rip it up by placing "hard to finish" cards.
You are a gold digger
Always help the other players.
Check out the players who are discarding cards often at the beginning of the game.
Never build too many crossroad paths so you are not blocked when a tunnel collapses (by a saboteur).
Never block players as long as you are not sure it are saboteurs.
Try to get the gold first if all looks fine for the other gold diggers.