If the runner is in the way and you have to make a throw that would have resulted in an out to get the call, you will hit the runner throwing directly to the bag. An earlier poster said it was the catcher's fault for throwing a rainbow to first, thus not getting the interference call.
I don't believe that is at all correct. The runner may not intentionally interfere with the thrown ball, or the fielder receiving the throw, but it is not the runner's responsibility to get out of the way of the throw.