It tends to depend on the size of the area in front of the "front row". At some venues they allow the first few rows (room permitting), others it's just the front row who are allowed to move to the stage/barrier.
Everyone else will stand up in front of their seat.
Then you get the "brave ones"!
My seated venue experiences are ...
Newcastle city hall 1992 - was totally crazy !! seats stood on and broken in a semi circle about 5 rows deep - was a wild night but not in a violent way.
same venue 1994 - a lot more sedate with heavier security - no broken seats and a hefty barrier .
same venue 2004 - really good gig - was about 4 rows back but made a dash for the front when the lights dimmed - was not stopped ! very passionate audience and very well humoured show ! no barrier and moz was actively encouraging stage invaders ! and he was obliged .. many times ! My favourite was the woman who popped a hat on his head and a scarf round his neck as she danced around him - hilarious! After being hoisted up i made a dash for moz .. only to be yanked around by security - firm but fair really! great gig !
sage , gateshead 2006 - No barrier at all , but as previously reported it was officially morrisseys' least favourite gig of the tour ! I was front row with elbows on stage the whole gig - came out to first of the gang and the audience ... just didn't do anything really ! I was jumping up and down and felt like a f***in prat as the whole front row were venue staff/mates and were all stuffed f***in shirts ! Not one person tried to get onstage and moz left with no encore to a room full of boos and jeers !
sunderland 2008 - lovely old style variety theatre ! massive pit infront of stage with quite a heavy handed security presence .. but it was a great gig even though moz had a bad throat ! a few attempts to press palms etc etc but no one got on to the stage .
other than that , with the exception of the albert hall 2001 and palladium every other gig has been standing (for me anyway) and they are by far the best for a moz gig (the smaller the better - Whitehaven was only 600 people and was to pretty much a school hall) got standing for 3 x 2009 so far - C'mon !!!!