[Arthur, unlike Merlin, has no idea wtf is going on or why he's stuck or what is happening and can only guess it's magic-related as he attempts to move from his spot - and it probably looks utterly ridiculous and somewhat flail-y.]
Merlin, if you're the one doing this I swear - ! [except if it were Merlin, he'd hear him laughing at this point and/or the little bastard would show up to grin at him. So. What the hell.] Stop it!