There is a third major definition of temperament: the slow-to-warm-up child. This child is perfectly happy and well adjusted, but he needs to proceed with new activities and interactions with new people at his own deliberate pace. This is perfectly normal. Pushing your shy 1-year-old to play with a new neighbor or to clamber onto the community fire truck may make him only more reluctant to try novel experiences.
If your child fits this description, you have to walk a fine line, respecting his initial reluctance to step forward and participate in life without inadvertently reinforcing his hesitation. Let him take a new experience at his own pace, but gently make him understand that if he waits too long, it may be too late for a ride on the swing or a taste of the new flavor of ice cream or a turn to play the drum. Whenever possible, let another child or another family member go first. If he sees you having fun as you cruise up the block on your new bicycle, he may be ready to ride in his safety seat (with a helmet) sooner than if you had tried to strap him in without showing him what a bike ride is all about.
Give your child time. If you are trying out a new Saturday-night babysitter, pick her up a half-hour early and let her spend some time with your child while you're getting ready to go out. Better still, have a new sitter come for the first time when you will be staying at home. Play with the sitter and your child for a while, then busy yourself with something nearby, and finally leave them alone together once it seems that your child is feeling comfortable.
Eventually, your respect for your child's individual style of behavior will boost his self-esteem, and this in turn may make him more eager for new experiences.