a={-1,2};
b={2,-3};
Graphics[{PointSize[Large],Point[a],Point[b]},Axes->True,PlotRange->4]

a={-1,2};
b={2,-3};
Manipulate[Show[{
Graphics[{PointSize[Large],Point[a],Point[b],{Red,Point[(1-t0)a+t0 b]},Text[Style["λ="<>ToString[Round[t0,.01]],FontSize->15,Bold,Red],(1-t0)a+t0 b+{.7,0}],Text[Style[ToString[b],FontSize->15],b-{.6,0}],Text[Style[ToString[a],FontSize->15],a-{.6,0}]},Axes->True,PlotRange->4],
ParametricPlot[(1-lambda)a+lambda b,{lambda,0,1},PlotStyle->Thick]
}],{{t0,0},-0.5,1.5,.01}]

f[t_]:={t,0,Sin[t]};
g[t_]:={t,2,Cos[t]};
Show[{
ParametricPlot3D[f[t],{t,0,2Pi}],
ParametricPlot3D[g[t],{t,0,2Pi}]
},PlotRange->All]

f[t_]:={t,0,Sin[t]};
g[t_]:={t,2,Cos[t]};
Show[{
ParametricPlot3D[f[t],{t,0,2Pi}],
ParametricPlot3D[g[t],{t,0,2Pi}],
ParametricPlot3D[(1-lambda)f[t]+lambda g[t],{t,0,2Pi},{lambda,0,1},PlotStyle->Thick]
},PlotRange->All]

f[t_]:={Cos[t],Sin[t],t};
g[t_]:={-Cos[t],-Sin[t],t};
Show[{
ParametricPlot3D[f[t],{t,0,2Pi}],
ParametricPlot3D[g[t],{t,0,2Pi}]
},PlotRange->All]

f[t_]:={Cos[t],Sin[t],t};
g[t_]:={-Cos[t],-Sin[t],t};
Show[{
ParametricPlot3D[f[t],{t,0,2Pi}],
ParametricPlot3D[g[t],{t,0,2Pi}],
ParametricPlot3D[(1-lambda)f[t]+lambda g[t],{t,0,2Pi},{lambda,0,1},PlotStyle->Thick]
},PlotRange->All]