Rule 4.03

Rule 4.03
When the ball is put in play at the start of, or during a game, all fielders other than the catcher shall be on fair territory.

(a) The catcher shall station himself directly back of the plate. He may leave his position at any time to catch a pitch or make a play except that when the batter is being given an intentional base on balls, the catcher must stand with both feet within the lines of the catcher’s box until the ball leaves the pitcher’s hand.

PENALTY: Balk.

(b) The pitcher, while in the act of delivering the ball to the batter, shall take his legal position;

(c) Except the pitcher and the catcher, any fielder may station himself anywhere in fair territory.


DISCUSSION
It is common for first baseman to have one foot in foul territory when holding a runner. Umpires are directed to instruct the first baseman to move into fair territory with both feet. While there is no penalty, refusal warrants ejection. [MLBUM]

A player may move into foul territory once the ball is put into play. [Jim Evans interp]

The Jaksa / Roder manual suggests that if a player positions himself in dead ball territory when the ball is put into play, the penalty should be:

1. if the umpire recognizes the violation, he should call time and direct the player to move into fair territory;

2. if a play is made involving a player in violation of the rule (usually a pickoff attempt), the penalty is a balk; or

3. if a pitch is made, the result stands unless the defense gains an advantage attributable to the violation.

However, the Jim Evans interpretation is that if an inning is started with less than eight players in fair territory, all action should be nullified. This is consistent with a PBUC interpretation from 1984 and at least one application during a MLB game. The game was inadvertently started without a center fielder. One pitch was thrown before the player took his position. The pitch was negated and the inning was restarted.
http://articles.latimes.com/1988-06-26/sports/sp-8197_1_time-white

Intentional Walk

A catcher is very rarely (read: never) called for a balk for stepping outside the catcher's box during an intentional walk. The determination of whether the catcher is in the catcher’s box should be made from the point that the pitcher releases the ball in his delivery. This rule should only be enforced when the catcher is attempting to take advantage of the rule by leaving the box to prevent runners from being able to steal during an intentional walk. [WMU]

Runners on second and third. Pitcher begins motion while catcher is outside box.
http://www.youtube.com/watch?v=8LnbL-WGWBE

Defensive shift.