1) Add starch 2) Add starch to 3) Become less flexible 4) Become rigid 5) Become stiff or stiffer 6) Brace for a shock 7) Brace oneself 8) Change 9) Exhibit stage fright 10) Get starched 11) Make stiff or stiffer 12) Necks might do it 13) Restrict 14) Tighten up 15) To become stiff 16) To make stiff 1) Rigidify 2) Set 3) Tense 4) Tense up 
• (v. t.) To make torpid; to benumb. • (v. t.) To make stiff; to make less pliant or flexible; as, to stiffen cloth with starch. • (v. t.) To inspissate; to make more thick or viscous; as, to stiffen paste. • (v. i.) To become stiff or stiffer, in any sense of the adjective.
Stiff'en transitive verb [ imperfect & past participle Stiffened ; present participle & verbal noun Stiffening .] [ See Stiff .] 1. To make stiff; to make less pliant or flexible; as, to stiffen cloth with starch. &...Stiff'en intransitive verb To become stiff or stiffer, in any sense of the adjective. « Like bristles rose my stiffening hair.» Dryden. « The tender soil then stiffening by degrees.» Dryden. « Some souls we see, Grow hard and stiffen
[v] - restrict 2. [v] - become stiff or stiffer 3. [v] - make stiff or stiffer
constrain verb restrict; `Tighten the rules`; `stiffen the regulations`
to make stiff.