ASA spells this out nicely in their rules. The definition of "interference" under Rule 1 flat out states that "contact is not necessary". It can't be any more plain than that!
Think about this...if contact was necessary, then while the fielder is fielding the ball the runner could legally stand right in front of her and wave her arms around, or get right next to her ear and scream at the top of her lungs, "Miss it!".
Sorry...ain't gonna happen...at least not without an out being called...
When you have a runner crossing in front of fielder who is fielding the ball, it can play out several different ways. Generally, a runner has the right to advance directly to the next base. However, she is also obligated to not impede the fielder's play on the ball. Simply passing in front of the fielder, or jumping over the ball, are not illegal actions.
But, what if...
- The runner does something other than "simply advance to the next base? Say, she purposely slows down and times it so that she is in front of the fielder to screen her out just as the ball arrives, or stops right in front of the fielder and "dances around" to make it look like she's trying to dodge the ball. In other words, she makes some action other than advancing to the next base that seems designed to "hinder/impede/confuse" the fielder.
Now, an interference call becomes viable.
- Suppose that the fielder is stationary, waiting for the ball to come to her, but the runner just happens to run in front of her. This most likely is not interference.
- Switch it around so that the fielder is charging the ball, but has to check up or abandon her play on the ball because the runner is in her way. This can be ruled as interference.
- One more...if the fielder runs so close to the fielder that it impedes her, this can be interference. The runner is obligated to give the fielder some room and does have the option of leaving the baseline to avoid the fielder.
You can see that on a play like this there can be a broad range of possibilities depending on the details. The final determination is at the decision of the umpire's judgment. You can only hope that his judgment is grounded in the actual playing rules and how they are interpreted. Saying that "contact is necessary for interference" would mean that his basic understanding of the rule is flawed!