Cuba intercity bus tickets - book in advance vs on spot [duplicate]

• Should I book Viazul bus tickets on Viazul.com in advance?
• There are not so many departure times - can tickets be sold out if I will come to a bus station and will try to buy a ticket few hours before departure?
• Is it possible to reserve a ticket in a station for a route which is between different cities than station?

There is similar question about Intercity transportation in Cuba but it is more about specific routes and Viazul alternatives - taxi colectivos.