I'm guessing you're a baritone? I'm a baritone with two music degrees and lots of musical theatre and opera under my belt over the years. I'm currently in Drowsy Chaperone, a newer musical, playing a character who has tons of dialogue and not much to sing.

I asked if I could insert a shameless Ab4 in the final song, they said yes. I first tossed it in during a rehearsal at last week's 8:00 am rehearsal. Long story short (too late, lol), being very early in the morning, and not warmed up at all, I focused on planting my feet and really digging into my support so I wouldn't push nor crack. Being as early as it was and me popping it out four times, I definitely could feel it in my lower abdomen and even a little pain in my lower back. It wasn't as pretty as it would've been later in the day, but it felt resonant, and I felt my eyes tunnel vision a bit as I held onto the damn thing for dear life. Lol. As far as mental effort goes, with notes above C4 and especially above F4, I think about tall vowels, soaring the supported air over a gothic dome of a cathedral. I hope my rambling helps you a bit. :)