Your Marth experience will probably help your Roy game a bit since you want to space Tipped Nair/Dtilt with Roy in Neutral other than that it's probably just lack of Roy playing time, I'm sure once you buy him and use him regularly the switch between him and Marth will become second nature.
I actually disagree. Roy's spaces with similar moves but definitely not in the same timing/same way.
Before jumping in, just try and get the spacing for neutral jumps. You want to hit with the edge of your sword for nair and dtilt, but for some of your other neutral tools like fair, you actually want to hit at the edge of the
sweetspot to maximize your safety. Roy's sweetspots reach out further than you might think. Marth doesn't really land with his fair or nair as much as Roy does.
The timing for fastfalling after you start your aerial is much different than Marth too, so practice trying to hit with the lowest parts of fair and nair, fastfalling at the fastest opportunity. Marth doesn't throw the moves out of his jumps as fast as Roy usually does. And, even then, Roy's approach is focused more on threatening with empty hops instead of throwing hitboxes out all the time.
