Contact is not a requirement for interference.
And simply running in front of the fielder isn't automatically interference. If a runner seems to time it out so she purposely gets right in front of the fielder just as the ball arrives, then it can be interference. Usually, it's pretty obvious when this is done intentionally. You'll see the runner stop, maybe do a stutter step like she's trying to avoid the ball, then jump right in front of the fielder at the last second. Or, even stop and do that stutter step thing right in front of the fielder!
If all the runner is doing is running in a straight line to the next base, then she may pass in front of the fielder without penalty. However...if the fielder is charging the ball, then has to stop or check up due to the presence of the runner, then you may have interference.