For those of you who do hand embroidery on quilt blocks, do you iron interfacing on the back of the fabric or do you embroidery on the top piece with a solid muslin fabric on the bottom. My Crabapple Hill patterns say to layer a muslin piece underneath the main fabric. Seems like that it could wrinkle up or not be smooth. Iron on interfacing, however, might bubble up. Just wondering what others do.

I don't do either. I just put it in a hoop and away I go.

Having a double layer would just make the embroidery more difficult to do.

I posted a pic of my Crabapple Hill block here.
http://www.quiltingboard.com/t-124926-1.htm

I used a good quilt weight fabric and did not add the muslin they called for. The fabric I used was Kona Cotton Snow.
I dont use iron on interfacing either as I want the end result to be flexible, not stiff.

I use very thin batting, Dream Select. It really helps to be able to hide the threads.

I did one of the Crabapple patterns and double the muslin. I basted it together as suggested in the pattern instructions. Turned out great. No need to worry about knots etc showing through from the back because it won't with the double thickness. Be sure to baste it good.

I also use muslin under the fabric. Then I serge the 2 fabrics together around all 4 sides. I've never had trouble with wrinkles, and nothing shows thru to the front.

I use two layers. Looks great and is not hard at all to embroider through both.

Me too.

i use the muslin back- have been doing it that way for 45 years- that's how we were taught way back when in 4-H
the layer of muslin does not wrinkle up- you place your embroidery square and a matching muslin square together and hoop them--and stitch-
that second layer keeps loose threads (or carry overs) from showing through on the front and adds stability and body to your piece.

using a fusable ... i've tried once---it was VERY DIFFICULT to stitch through it- caused sore fingers and (i only did one block that way thank goodness- cause i felt it ruined one block)
my granddaughter's all love to embroider and i've taught them to back their work with muslin too