Jacket interfacing....where should it go?

Use armoweft fusible for entire front of jacket, under collar, pocket welts and as this is a lighter weight coating, on facings and upper collar.
Use a non fuse shapewell or cotton muslin cut on the bias for back 'saddle' which is stay stitched around armhole neckline and shoulders. The lower edge which isn't attached should be pinked so a press line won't show on outside.
Sleeves have a small amount of interfacing around the head and lower edge, which helps to give it a tailored look when finished.
You can also build up a breast plate using several overlaying layers from front shoulder around armhole as shown, this stops garment collapsing around front armhole.
After applying interfacing, pin pattern back on each section and trim if any distortion has occurred. Mark all necessary points, darts, pocket lines and especially ' the dot' ( stitching stop point at knotch collar junction on front and facing neckline).
I like to stay stitch the lapel roll- line to prevent stretching.