function game_update()
 -- usual gameplay update code...
end

function game_draw()
 -- usual draw code...
end

-- gets called from title screen
function start_game()
 -- do some init...

 _update=game_update
 _draw=game_draw
end

function game_update()
 -- usual gameplay update code
 -- ...

 -- dpad left, for example
 if btnp(0) then
  -- some kind of npc detection code
  local npc=get_npc_at(player.x-1,player.y)
  if npc then -- not nil means there was an npc
   talk_to(npc)
  else
   -- normal move logic
   -- check for walls etc
   player.x-=1
  end
 end

 -- etc...
end

function talk_to(npc)
 dialogue = npc.dialogue -- a list of "things to say"
 d_step = 1
 _update=dialogue_update
 _draw=nil -- no need to constantly draw at 30/60 fps
 dialogue_draw()
end

function dialogue_update()
 if btnp(4) then
  d_step+=1 -- next chunk of speech

  if d_step>#dialogue then -- end of conversation
   -- back to the usual gameplay
   _update=game_update
   _draw=game_draw
   return
  end

  dialogue_draw()
 end
end

function dialogue_draw()
 game_draw() -- underlying game graphics as backdrop
 local ds=dialogue[d_step] -- data 

 -- draw character "talking head" picture from packed spr args
 -- having this per step lets you move things around the screen
 -- or have a conversation involving multiple npcs
 spr(unpack(ds.spr))

 -- dialogue textbox
 rectfill(ds.x0,ds.y0,ds.x1,ds.y1,ds.bgcolor)
 rect(ds.x0,ds.y0,ds.x1,ds.y1,ds.bordercolor)
 print(ds.text,ds.tx,ds.ty,ds.tcolor)
end