We will be in Spain in September, and plan to do the Seville-Lisbon leg by bus. Can anyone give the time schedules, and recommendations about this journey ? Does one need to book in advance ?

not sure about direct buses but

there are some daily buses Seville-Faro and then Faro is but a few hours by train from Lisbon.

DAMAS/EVA joint service on bus

DAMAS (Huelva Spain) 34.959.256.900
EVA (Faro) 351.289.899.700/1

Alsa Bus has two daily departures from Plaza de Armas bus station to Lisbon, 3 pm and 11.59 pm. About 6 hours, 40 euro one-way.
Timetable and online booking (in English) at http://www2.alsa.es/portal/index.asp?lang=en

Thanks Alec, your information was very helpful. Are tickets usually available a few days in advance, or should one book well before time ?

I have travelled on Viajes Julia (DBA Eurolines): They operate about 5-days weekly. I reserved in advance (Bus from Sevilla's Plaza de Armas Terminal was pretty full).

Sorry, I don't remember the cost but it took about 8-hours. That's considering that Portugal is an hour earlier than Spain.

http://www.Eurolines.es