The primary biblical insight into the role of the deacon appears in Acts 6:1-7, where the apostles are overburdened by caring for needy widows in the community and the task is appointed to the deacons. I believe this is more of a descriptive than prescriptive scenario and the role of a deacon is that of a ‘servant’ as it is translated which can manifest in just about any capacity.
Deacons are greeted as church leaders who work closely with the elders (Philippians 1:1). The Bible offers a list of requirements for the deacons as well that is virtually identical to that of the elders (1 Timothy 3:8-13). Though the ability to teach is not described as a requirement for deacons I don’t believe that this limits them from preaching as they too are mandated to ‘hold to the deep truths of the faith’ (1 Timothy 3:9), and one of the first deacons, Stephen, preached to his death (Acts 6-7).
* I’ve got an image of a happy Starbucks employee because deacons first appeared in the Scriptures waiting on tables.