"So it's really nice after about a year and a half to get back on stage and flex those old muscles"

Michael C. Hall is referring to the happiness of going back to the phase after a long absence. He is revealing his enjoyment to be able to carry out once again and to use the abilities he has established over the years. He is also highlighting the value of practice and sharpening one's craft. He is recommending that even after a long break, the abilities and capabilities he has established over the years are still there and can be used to create a great performance. He is also expressing his gratitude for the opportunity to carry out again and to be able to utilize his talents. He is highlighting the importance of taking some time to practice and hone one's craft, and the pleasure of having the ability to utilize those abilities to develop something special.